This invention relates to the analysis of samples which are caused to flow along a flow channel. The method and apparatus of the invention are particularly well suited for use in the automatic analysis of samples of clinical interest, but are also well suited for use in the automatic analysis of agricultural, pharmaceutical and industrial samples.
Various types of analytical apparatus are known. One type of apparatus is known as a discrete analyser, and in this apparatus a sample is placed in an individual container where, in general, it remains for the duration of the analytical procedure which is carried out. An advantage of this arrangement is that each discrete sample can be individually labelled, so that confusion between different samples is minimised.
A further type of analyser mixes a sample under investigation with a reagent by injecting them into a chamber from which the reacting mixture is passed to a measurement cell. The mixture is held in this cell for a short time while a measurement, for example a photometric measurement, is carried out.
In a further type of apparatus, samples travel continuously in a carrier stream moving along a narrow flow channel which is designed so that each individual sample retains its integrity with respect to adjacent samples. In one system, slugs of air are used to separate individual samples in the moving stream. In another system, known as flow injection analysis, the carrier medium is the reagent with which the individual samples are to react. A precisely measured sample is injected into the flowing reagent stream at a given point. Injection can take place with a syringe, e.g. through a septum or by means of a rotary valve. In the latter case, the port in the rotary valve constitutes the measuring device; a slight excess of sample is injected through the port which is then rotated to deliver its contents into the flowing reagent stream. The injected slug of sample remains coherent as it passes along the narrow tubing which constitutes the flow channel, although the sample becomes elongated. The sample mixes with reagent by radial diffusion from the boundary layer. Successful operation of this flow injection analysis technique therefore requires that turbulent flow is avoided. This arrangement offers advantages over the system using air-segmented samples, in that it allows more rapid sampling rates without adverse effects on the accuracy of the analytical results obtained. The system does, however, have the disadvantage that it is wasteful of reagent, which in many cases will be an expensive commodity.